Full diagnostics for the drives were designed by system integrator Interstates Control Systems, Sioux Center, Iowa. The enhanced diagnostic software reduces manpower and physical diagnostic checks. This reduces REG’s overall repairs.
Scott Kingery, process engineer at REG, says the PACs primarily monitor the each motor’s current draw in amperes and the run time of all motors. “Every few minutes, motor readings are logged into the data historian in case any problems come up,” says Kingery. “If a motor fails, we can look at the historical data to get an idea of why it failed.”
Figure 2: Renewable Energy Group (REG) uses motor-driven pumps to make biodiesel. These pumps are located on the bottom of the glycerin methanol stripper. REG uses built-in diagnostics and reports few motor problems.
So far, very few have. Kingery says this is because of the newer technology. “In the old days, motors would overload, heat up, pull too much current and ‘kick out’ or burn up,” he explains. “With the new technology the drive controller spots an overload fault condition almost instantly and reacts accordingly. Today’s systems have shut-down parameters that protect the motor from stress and wear. The PAC identifies the fault, and the plant operators can reset it using an HMI, thus avoiding going out into the plant and physically hitting the reset button. We’ve rarely lost a motor in two and half years.”
Not everybody is in love with modern motor diagnostics.
Eaton’s Krug is aware of the overload problem in motors. “Some users don’t want their motor or load coming offline for anything but a motor overload,” he explains. “When the revenue of throughput is more valuable than motors and loads, one can understand this logic.” So Krug suggests using modern overload relays.
“Many advanced overload relays can measure current and voltage,” he explains. “The combination of information gathered from current and voltage and the processing power available in today’s overload relays can provide protection. For example, Eaton’s Motor Insight relay uses line voltage and motor current to provide 11 protective settings, such as volts, power consumption, current, voltage imbalance, power factor, frequency, ground fault current and thermal capacity, and provide protection from overloads and poor line conditions. Unlike more complex diagnostic solutions, these overload relays install just like bi-metallic overloads, and arrive with default protective settings,” he says.
Jim Reizner of Procter & Gamble also does not use built-in diagnostics. “We do various things in the area of motor preventive maintenance,” he says. “We do thermography of connections and sometimes of motor bearings. We megger [apply a meg ohm meter] motors—rarely as a diagnostic tool—but more generally just before installation to ensure the motor is OK.”
“Motor bearing vibration monitoring is our most common method of predicting motor failure,” he says. “In most of our plants, this is done as a rounds-based task, and then only for specified larger and critical motors. Some larger motors have sensors installed to monitor vibration, and we have experience using some newer wireless vibration sensors, most usually from VenTek.”
Vibration analyzers are widely available from several vendors. As Reizner says, checking motors is typically done manually, but some systems can be installed permanently. Like the built-in drive diagnostics noted above, they can connect to a process control system via the plant network or wireless.
“Some of our plants use Baker motor testers,” Reizner continues. “These are excellent, but they require an amount of knowledge, training and time that many plants do not seem able to afford.”
What Reizner wants is affordable and simple diagnostics. “Diagnostics would need to be broader than only in drives, as the vast majority of motors are operated off of motor contactors/starters, not drives,” he says.
No Magic Bullet?
Dr. Howard Penrose, general manager of All-Test Pro Division of BJM Corp., says, “There’s been a persistent misconception that there is a ‘magic bullet,’ in the form of a condition-based monitoring (CBM) instrument that will provide all the information you need to evaluate the health of your electric motor system.”
This misconception, he says, mostly comes from the vendors of those CBM systems. In reality, he says, there is no one CBM device that does it all. Instead, you need a multiple-technology approach to keep your motors humming. His laundry list of motor diagnostic tools include those that can test for:
- DC high potential
- Surge comparison
- Polarization index
- Resistance between windings
- Motor circuit analysis (MCA)
- Voltage and current
- Motor current signature analysis (MCSA).
"Infrared and vibration are normally used with each other, with great success,” Penrose says. “However, they miss a few common problems or will only detect them in the late stages of failure. Surge and high-potential testing will only detect some winding and insulation to ground faults.
“The newest and most effective approach has been vibration, infrared, MCA and/or MCSA,” he advises. “As found in a recent study, 38% of motor system tests involving only vibration and/or infrared see a significant return on investment. This number jumped to 100% in systems that used a combination of MCA/MCSA along with vibration and/or infrared.”
Dave Polka, an instructor at ABB, says simple maintenance techniques can keep variable-frequency drives (VFDs) running reliably. Keep it clean, keep it dry and keep the connections tight, he says. “Dust on VFD hardware can cut airflow, while dust on electronics can cause malfunctions. Moisture can corrode circuit boards. Bad connections eventually lead to arcing, nuisance overvoltage faults, clearing of input fuses or damage to power components.”
While REG’s engineers rely on their Rockwell Automation diagnostics to keep their motors humming, they still manually test all new motors before installation. And Scott Kingery says they still have a full set of meters, meggers, gauges and equipment to diagnose a motor if one ever does fail.
When it comes to keeping motors humming, it pays to be as careful as possible.
Rich Merritt is a Control contributing editor.